1. Field of the Invention
The present invention relates to an automotive brake fluid pressure control apparatus employed in brake pressure control, for example in traction control during acceleration slippage and in antiskid control during braking operation, to cause to enhance motion characteristic of a vehicle.
2. Related Art
Conventionally, as shown for example in FIG. 7, a hydraulic circuit of a structure which drives hydraulic pumps P1 and P2 with a hydraulic motor MR has been considered as a hydraulic circuit of an automotive brake fluid pressure control apparatus employed in traction control during acceleration slippage or the like.
This hydraulic circuit performs the traction control for a vehicle (FF) of a front-wheel drive and a front-mounted engine, and valves and the like to perform normal braking (in response to depression of a brake pedal BP by a driver) and the traction control are disposed in a conduit of brake fluid reaching from a master cylinder (M/C) to wheel cylinders (W/C) of left and right front wheels FL and FR. In specific terms, this includes a master-cylinder cutoff valve (SM valve) which cuts off brake fluid from the M/C, a reservoir cutoff valve (SR valve) which cuts off communication from a reservoir R on the M/C side to the intake side of the hydraulic pumps P1 and P2, and additionally electromagnetic-type valves V1 and V2 which perform holding and release of high-pressure brake-fluid pressure applied to the W/Cs.
Accordingly, in a case of executing traction control with this hydraulic circuit (hereinafter described with reference to the FL wheel), braking force is controlled by performing holding and release of brake-fluid pressure with the hydraulic control valves V1 and V2 in a state after the conduit has been interrupted by the SM valve and moreover the conduit has been communicated by the SR valve, and along with this, a motor relay MR has been switched on to drive the hydraulic pump P1, which takes in the W/C-side brake fluid to rise pressure thereof. The hydraulic control valve V1 is a pressure-increasing valve which is switchable to a pressure-increasing position to communicate the conduit and to a holding position to interrupt the conduit, and the other hydraulic control valve V2 is a pressure-reducing valve which is switchable to a pressure-reducing position to communicate the conduit in order to expel brake fluid within the W/C and to a holding position to interrupt the conduit.
The pressure-increasing valve V1 is normally at the pressure-increasing position, and an electromagnetic solenoid thereof is driven by conductance from a brake controller (not illustrated) and switched to the holding position. Additionally, the pressure-reducing valve V2 is normally in an interrupted state, and an electromagnetic solenoid thereof is driven by conductance from the brake controller to change into a communicated state and expel brake fluid within the W/C. In this way, three modes--i.e., pressure-increasing, holding, and pressure-reducing--can be switched by the brake controller.
Accordingly, as is disclosed, for example, in Japanese Patent Application Laid-open No. 60-56662 as control of brake-fluid pressure, in a case of pressure increase, a ratio of time to increase pressure of brake fluid per a uniform time is determined in accordance with an amount of change in vehicle-wheel slippage, and duty control where the pressure-increasing valve is repeatedly switched between the pressure-increasing position and holding position in accordance with the ratio of time is performed and thus the brake fluid pressure is increased. Simultaneously, in a case of pressure reduction, a ratio of time to reduce pressure of brake fluid per a uniform time is determined in accordance with an amount of change in vehicle-wheel slippage, and duty control where the pressure-reducing valve is repeatedly switched between the pressure-reducing position and holding position in accordance with the ratio of time is performed and thus brake fluid pressure is reduced.
However, in a case of attempting to initiate the traction control, for example, at a time of low temperature, there existed a problem wherein traction control could not be performed favorably due to a reason of worsening of discharge capacity of the pump P1 and insufficiency of amount of fluid discharge or the like. This problem will be described with reference to FIG. 8. FIG. 8 is a time chart indicating a state of a motor relay MR to cause to drive a pump P, a state (herein either of three modes of pressure increase, holding, or pressure reduction) of the electromagnetic solenoid to cause to drive the pressure-increasing valve V1 and pressure-reducing valve V2, and brake-fluid pressure of a wheel cylinder.
Briefly, because viscous resistance increases at a time of low temperature, discharge capacity of the pump P1 worsens and resistance when brake fluid passes through a needle valve becomes large, and so when a time of pressure increase is considered, an actual amount of increase in brake-fluid pressure is small even when in a pressure-increasing mode for a pressure-increasing time identical to when at a normal temperature. In the time chart of brake-fluid pressure in FIG. 8, a solid line indicates when at a normal temperature and a broken line indicates when at a low temperature. As is understood from these, the pressure-increase gradient becomes smaller and convergence of slippage deteriorates as a result. Additionally, when time at normal temperature (solid line) and time at a low temperature (broken line) even during pressure reduction are compared, not unexpectedly the pressure-reduction gradient of brake-fluid pressure at a time of low temperature even when in the pressure-reducing mode is small for the same pressure-reduction time, and speed loss due to delay in brake-fluid pressure reduction occurs.
Additionally, as a cause of worsening of discharge capacity of the hydraulic pump P1 when at a low temperature, not only the increase in viscous resistance of brake fluid but also, for example, the decline of a battery voltage can be considered. That is, the hydraulic pump P1 is driven by a battery, if battery voltage declines due to low temperature, discharge capacity not unexpectedly worsens.